Browsing by Author "Cakmak, A. O."

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Open Access
Asymmetric Fabry-Perot-type transmission in photonic-crystal gratings with one-sided corrugations at a two-way coupling
(American Physical Society, 2012-11-29) Serebryannikov, A. E.; Alici, K. B.; Magath, T.; Cakmak, A. O.; Özbay, Ekmel
Strongly asymmetric Fabry-Perot-type transmission arising at the two-way coupling has been studied in the case of normal incidence for slabs of two-dimensional photonic crystals (PCs) with one-sided corrugations that are made of linear isotropic materials. Comparing to the scenario of unidirectional transmission known for the structures with broken spatial inversion symmetry that requires zero order being uncoupled, in the studied mechanism zero order is either the sole order or one of the orders that may be coupled to a Floquet-Bloch mode. Contrary to the earlier studies of asymmetric transmission at the coupled zero order, structures with nondeep corrugations are considered, which allow one to combine Fabry-Perot-type total-transmission maxima with diffractions in a desired way. At a proper choice of PC lattice and corrugation parameters, higher orders can dominate in Fabry-Perot-type transmission at the noncorrugated-side illumination and also at the total-transmission maxima, whereas only zero order contributes to the transmission at the corrugated-side illumination. As a result, strong asymmetry can be obtained without uncoupling of zero order but it invokes the unidirectional contribution of higher orders. The presented results show that the entire structure can be approximately decomposed into the two independent, regular and grating (nonregular), parts whose contributions to the transmission are additive. Multiple asymmetric transmission maxima can coexist with a rather high equivalent group index of refraction. Possible applications of the studied transmission mechanism are discussed.
Open Access
Asymmetric transmission in prisms using structures and materials with isotropic-type dispersion
(Optical Society of America, 2015) Gundogdu, F. T.; Serebryannikov, A. E.; Cakmak, A. O.; Özbay, Ekmel
It is demonstrated that strong asymmetry in transmission can be obtained at the Gaussian beam illumination for a single prism based on a photonic crystal (PhC) with isotropic-type dispersion, as well as for its analog made of a homogeneous material. Asymmetric transmission can be realized with the aid of refraction at a proper orientation of the interfaces and wedges of the prism, whereas neither contribution of higher diffraction orders nor anisotropic-type dispersion is required. Furthermore, incidence toward a prism wedge can be used for one of two opposite directions in order to obtain asymmetry. Thus, asymmetric transmission is a general property of the prism configurations, which can be obtained by using simple geometries and quite conventional materials. The obtained results show that strong asymmetry can be achieved in PhC prisms with (nearly) circular shape of equifrequency dispersion contours, in both cases associated with the index of refraction 0 < n < 1 and n > 1. For the comparison purposes, results are also presented for solid uniform nonmagnetic prisms made of a material with the same value of n. It is shown in zero-loss approximation that the PhC prism and the ultralow-index material prism ( 0 < n < 1) can replace each other in some cases without affecting the scenario of asymmetric transmission. Moreover, the PhC prism and the solid dielectric prism can show the same scenario at n > 1. Possible contributions of scattering on the individual rods and diffraction on the wedge to the resulting mechanism are discussed. Analogs of unidirectional splitting and unidirectional deflection regimes, which are known from the studies of PhC gratings, are obtained in PhC prisms and solid uniform prisms, i.e. without higher diffraction orders. ©2015 Optical Society of America.
Open Access
Bilayer metamaterial: analysis of left-handed transmission and retrieval of effective medium parameters
(Institute of Physics Publishing Ltd., 2007) Guven, K.; Cakmak, A. O.; Caliskan, M. D.; Gundogdu, T. F.; Kafesaki, M.; Soukoulis, C. M.; Özbay, Ekmel
We report an experimental and numerical analysis of a planar metamaterial designed for normal-to-plane propagation, and operating at microwave frequencies. The metamaterial consists of cutwire and wire patterns, which are arranged periodically on both sides of a dielectric layer, in the form of a bilayer. The left-handed transmission band of the metamaterial is demonstrated experimentally. The effective index of refraction retrieved from the S parameters is found to be negative within this transmission band. An independent negative refraction experiment supports the existence of the negative index of refraction for the metamaterial.
Open Access
Dispersion irrelevant wideband asymmetric transmission in dielectric photonic crystal gratings
(Optical Society of America, 2012-11-22) Serebryannikov, A. E.; Colak, E.; Cakmak, A. O.; Özbay, Ekmel
Wideband suppression of zero order and relevant strongly asymmetric transmission can be obtained in photonic crystal gratings that are made of linear isotropic materials and show the broken structural (axial) symmetry, even if zero diffraction order may be coupled to a Floquet-Bloch (FB) wave at the incidence and exit interfaces. The studied mechanism requires that the peculiar diffractions at the corrugated exit interface inspire strong energy transfer to higher orders, including those not coupled to an FB wave. At the opposite direction of incidence, transmission due to zero and some higher orders that may be coupled at the corrugated input interface can vanish. This leads to the alternative scenario of wideband unidirectional transmission, which itself does not need but can coexist with the other scenario based on the merging of asymmetric diffraction and dispersion of the FB mode.
Open Access
Enhanced transmission through a subwavelength aperture using metamaterials
(AIP Publishing LLC, 2009-08-04) Cakmak, A. O.; Aydin, K.; Colak, E.; Li, Z.; Bilotti, F.; Vegni, L.; Özbay, Ekmel
We report an enhanced transmission through a single circular subwavelength aperture that is incorporated with a split ring resonator (SRR) at the microwave regime. Transmission enhancement factors as high as 530 were observed in the experiments when the SRR was located in front of the aperture in order to efficiently couple the electric field component of the incident electromagnetic wave at SRR’s electrical resonance frequency. The experimental results were supported by numerical analyses. The physical origin of the transmission enhancement phenomenon was discussed by examining the induced surface currents on the structures.
Open Access
Experimental study of broadband unidirectional splitting in photonic crystal gratings with broken structural symmetry
(American Institute of Physics, 2013-04-15) Colak, E.; Serebryannikov, A. E.; Cakmak, A. O.; Özbay, Ekmel
It is experimentally demonstrated that the combination of diode and splitter functions can be realized in one broadband reciprocal device. The suggested performance is based on the dielectric photonic crystal grating whose structural symmetry is broken owing to non-deep corrugations placed at one of the two interfaces. The study has been performed at a normally incident beam-type illumination obtained from a microwave horn antenna. The two unidirectionally transmitted, deflected beams can show large magnitude and high contrast, while the angular distance between their maxima is 90° and larger. The dual-band unidirectional splitting is possible when using TM and TE polarizations. © 2013 American Institute of Physics
Open Access
Frequency dependent steering with backward leaky waves via photonic crystal interface layer
(Optical Society of America, 2009) Colak, E.; Caglayan, H.; Cakmak, A. O.; Villa, A. D.; Capolino, F.; Özbay, Ekmel
A Photonic Crystal (PC) with a surface defect layer (made of dimers) is studied in the microwave regime. The dispersion diagram is obtained with the Plane Wave Expansion Method. The dispersion diagram reveals that the dimer-layer supports a surface mode with negative slope. Two facts are noted: First, a guided (bounded) wave is present, propagating along the surface of the dimer-layer. Second, above the light line, the fast traveling mode couple to the propagating spectra and as a result a directive (narrow beam) radiation with backward characteristics is observed and measured. In this leaky mode regime, symmetrical radiation patterns with respect to the normal to the PC surface are attained. Beam steering is observed and measured in a 70 degrees angular range when frequency ranges in the 11.88-13.69GHz interval. Thus, a PC based surface wave structure that acts as a frequency dependent leaky wave antenna is presented. Angular radiation pattern measurements are in agreement with those obtained via numerical simulations that employ the Finite Difference Time Domain Method (FDTD). Finally, the backward radiation characteristics that in turn suggest the existence of a backward leaky mode in the dimer-layer are experimentally verified using a halved dimer-layer structure. (C) 2009 Optical Society of America
Open Access
High efficiency of graded index photonic crystal as an input coupler
(AIP Publishing LLC, 2009) Cakmak, A. O.; Colak, E.; Caglayan, H.; Kurt, H.; Özbay, Ekmel
A graded index photonic crystal (GRIN PC) configuration was placed at the input side of a photonic crystal waveguide (PCW) in order to efficiently couple the light waves into the waveguide. We compared the transmission efficiencies of light in the absence and presence of the GRIN PC structure. We report a significant improvement in coupling when the GRIN PC is incorporated with the PCW. The intensity profiles were obtained by carrying out the experiments at microwave frequencies. Finite difference time domain based simulations were found to be in good agreement with our experimental results.
Open Access
Multiple slow waves and relevant transverse transmission and confinement in chirped photonic crystals
(OSA Publishing, 2014-09-02) Serebryannikov, A. E.; Cakmak, A. O.; Colak, E.; Caglayan, H.; Kurt, H.; Özbay, Ekmel
The dispersion properties of rod-type chirped photonic crystals (PhCs) and non-channeled transmission in the direction of the variation of structural parameters from one cell of such a PhC to another are studied. Two types of configurations that enable multiple slow waves but differ in the utilized chirping scheme are compared. It is demonstrated that the multiple, nearly flat bands with a group index of refraction exceeding 180 can be obtained. For these bands, transmission is characterized by multiple narrow peaks of perfect transmission, strong field enhancement inside the slab, and large values of the Q-factor. Among the bands, there are some that show negative phase velocity. Symmetry with respect to the slab mid-plane must be kept in order to obtain constructive interferences that are necessary for reflection-free transmission. It is shown that 15 and more slow wave bands can be obtained in one configuration. The corresponding transmission peaks are well separated from each other, being the only significant feature of the transmission spectrum, while the Q-factor can exceed 105. The observed features are preserved in a wide range of the incidence angle variation. They can be used for tuning the locations and spectral widths of the transmission peaks. Some comparisons with the chirped multilayer structures have been carried out. ©2014 Optical Society of America.
Open Access
Near-field light localization using subwavelength apertures incorporated with metamaterials
(Elsevier, 2012-03-12) Ates, D.; Cakmak, A. O.; Özbay, Ekmel
We report strong near-field electromagnetic localization by using subwavelength apertures and metamaterials that operate at microwave frequencies. We designed split ring resonators with distinct configurations in order to obtain extraordinary transmission results. Furthermore, we analyzed the field localization and focusing characteristics of the transmitted evanescent waves. The employed metamaterial configurations yielded an improvement on the transmission efficiency on the order of 27 dB and 50 dB for the deep subwavelength apertures. The metamaterial loaded apertures are considered as a total system that offered spot size conversion ratios as high as 7.12 and 9.11 for the corresponding metamaterial configurations. The proposed system is shown to intensify the electric fields of the source located in the near-field. It also narrows down the electromagnetic waves such that a full width at half maximum value of λ/29 is obtained.
Open Access
Simultaneously opening transmission channels with negative and positive phase velocities for the stacked subwavelength apertures in fishnet metamaterials with hybrid unit lattices
(S P I E - International Society for Optical Engineering, 2012-10-05) Cakmak, A. O.; Colak, E.; Özbay, Ekmel
Hybridization of the unit lattice along the propagation direction was demonstrated to produce a negative-phase-velocity transmission band in the absence of the contributions from the higher diffraction orders for a stacked metallic fishnet grid with subwavelength apertures. This extraordinary transmission band is governed by the stacked resonators. The hybridized unit lattice configurations are not just slight modifications of the configurations with homogenous unit lattices. The volumetric proportions of different dielectric media are a key factor in the partitioned unit lattice for the estimation of the stacking and coupling effects between the resonators. The contribution of the coupling mechanisms enhances the transmission results almost by the same factor for the investigated lattice separations along the propagation direction in hybrid unit cells while the densely stacked resonators yield much higher transmission results, both around the regarding extraordinary transmission band that is associated with the negative phase velocity. A positive-phase-velocity transmission band was also exhibited by the hybridized unit lattice configuration when combined with a cavity resonator. Experimental transmission results of the hybrid configuration supported the theoretical predictions. The hybrid configurations are scalable to the near-infrared regime.
Open Access
Spatial filtering using dielectric photonic crystals at beam-type excitation
(American Institute of Physics, 2010-12-06) Colak, E.; Cakmak, A. O.; Serebryannikov, A. E.; Özbay, Ekmel
Spatial filtering is demonstrated at beam-type excitations by utilizing finite thickness slabs of two-dimensional dielectric photonic crystals (PCs) showing exotic Fabry-Perot resonances that are preserved over a wide range of variation of the incidence angle. Bandstop and dual-bandpass filtering effects are illustrated theoretically and the corresponding filters are validated in the microwave experiments by using square-lattice PCs. It is shown that the basic transmission features that were observed earlier for a plane-wave illumination are also recognizable at beam-type excitations. The proposed spatial filtering mechanism exhibits directional beaming. The desired widths and the locations of the passbands and stopbands are attainable in the angle domain with a proper choice of the operating frequency for the given excitation characteristics.
Open Access
Split-ring-resonator-coupled enhanced transmission through a single subwavelength aperture
(American Physical Society, 2009) Aydin, K.; Cakmak, A. O.; Sahin, L.; Li, Z.; Bilotti, F.; Vegni, L.; Özbay, Ekmel
We report the enhanced transmission of electromagnetic waves through a single subwavelength aperture by using a split-ring resonator (SRR) at microwave frequencies. By placing a single SRR at the near field of the aperture, strongly localized electromagnetic fields are effectively coupled to the aperture with a radius that is 20 times smaller than the resonance wavelength (r/λ=0.05). We obtained 740-fold transmission enhancement by exciting the electric resonance of SRR. A different coupling mechanism, through the magnetic resonance of SRR, is also verified to lead to enhanced transmission.
Open Access
Transmission measurements of a new metamaterial sample with negative refraction index
(Elsevier, 2010) Sabah, C.; Cakmak, A. O.; Özbay, Ekmel; Uckun, S.
We presented the microwave experiments with a new metamaterial composed of triangular split ring resonators (TSRRs) and wire strip at microwave regime. The transmission measurements were performed in free space for two LHM samples which have different number of TSRRs and wire strips. The experimental results show that the left-handed transmission peak stands in the frequency band where both the permittivity and permeability are negative. It is also observed that left-handed transmission band can be shifted if the number of TSRRs and wire strips are changed.